This invention relates to an improved capacitor structure and a method of making it.
A typical capacitor, such as the capacitor 150 illustrated in FIG. 15, comprises two electrodes 152, 154 separated by a distance 156 with air or some dielectric material 158 between the electrodes. The electrodes or plates 152, 154 may take a variety of shapes and may, for example, be wound up into a cylindrical shape. The main purpose for this is to increase the size of the electrodes 152, 154. Since the capacitance is dependent on the size of the electrodes, dielectric constant, and the separation between the electrodes, much work has been put into increasing the size of the electrodes and minimizing the separation between the electrodes.
In the field of integrated circuits, capacitors are implemented to perform capacitive functions. A common approach is to use a poly-poly capacitor comprising two polysilicon electrodes of highly doped silicon separated by a dielectric. Due to the nature of semiconductors, capacitors in the integrated circuit arena typically have planar electrodes. The capacitance is therefore dictated by the three factors mentioned above, namely the size of the electrodes and the separation between the electrodes. One drawback of using poly-poly capacitors is the need for the deposition of a special poly layer, requiring additional process steps and thus increased cost.
An additional problem encountered in the manufacturing of capacitors in integrated circuits is caused by process variations which may result in different thicknesses electrodes and variations in the thickness of the dielectric layer. It will be appreciated that such changes will invariably effect the capacitance. More specifically, a ten percent change in the dielectric thickness will correlate directly to a ten percent variation in capacitance.
Ideally, therefore, it is desirable to have a capacitor structure that displays the maximum capacitance, stability in spite of process variations, and avoids, as much as possible, the need for additional process steps.
According to the invention, there is provided a capacitor structure, and a method of creating such a structure in a semiconductor device, comprising at least two electrodes separated by dielectric, wherein the electrodes have a comb-like structure. Preferably the electrodes are made of metal. The electrodes may lie in the same plane with fingers of the comb-like structures extending between one another in alternating fashion. Additional electrode pairs may lie on different planes separated by one or more dielectrics extending between the planes. Electrodes of the same polarity may be aligned with one another so that the fingers of electrodes of the same polarity lie in the same vertical plain, separated by the dielectric. Instead, the electrodes of the same polarity lying in different planes may be horizontally shifted with respect to one another which has the effect of increasing the side-wall coupling capacitance. This also has the added benefit of making the structure more resistant to parameter variations due to process variations. By optimizing the relative shift between electrodes in one plane relative to those in another plane, changes in capacitance value due to variations in inter-dielectric thickness can be largely eliminated.
In a preferred embodiment, more than two electrodes are used to define the capacitor.
Instead of the using substantially parallel extending fingers, the shape of the electrode may, instead take the form of a spiral structure or other shape that defines one or more elongate electrode elements to increase side-wall coupling capacitance and permit the flexibility of shifting one electrode relative to another one.
Preferably the electrodes are created from the metal interconnect layers commonly found in integrated circuits for forming the metal interconnect lines.